1. Field of the Invention
The present invention relates to a semiconductor device comprising at least a semiconductor film accompanying lattice distortion therein, and to a method for manufacturing the same.
2. Description of the Prior Art
Methods for fabricating polycrystalline semiconductor films to use in polycrystalline semiconductor devices include (1) depositing films in the temperature range of from 550 to 900xc2x0 C. using low pressure CVD (chemical vapor deposition), (2) thermally crystallizing semiconductor films having deposited by low pressure CVD, by annealing the films in the temperature range of from 550 to 650xc2x0 C. for a duration of from several to several tens of hours, and (3) thermally crystallizing semiconductor films having deposited by plasma-enhanced CVD, by annealing the films in the temperature range of from 550 to 650xc2x0 C. for a duration of from several to several tens of hours.
However, in depositing a non-single crystal semiconductor film, a reduced pressure CVD method fails to deposit uniformly the film over a large area on a substrate, and a plasma CVD method takes too long a time to deposit the film to a sufficient thickness.
There is also known a method of fabricating thin-film transistors using an amorphous silicon (a-Si) film having deposited by sputtering in the presence of hydrogen, however, the electric characteristics of the resulting thin film are so poor as to yield, e.g., an electron mobility of 0.1 cm2/Vsec or even lower.
If a non-single crystal semiconductor film is deposited by a sputtering method under an atmosphere free from hydrogen, on the other hand, the resulting film suffers segregation of silicon atoms. Furthermore, it is known that such a film would not undergo thermal crystallization at 700xc2x0 C. or at any temperature below 700xc2x0 C., due to the incorporation of impurities such as argon and oxygen atoms or to the lack of hydrogen, or due to both.
An object of the present invention is to provide a semiconductor device improved in device characteristics.
Another object of the present invention is to provide a method for fabricating a semiconductor device improved in device characteristics.
Further another object of the present invention is to provide a semiconductor device comprising a microcrystalline semiconductor film having a low barrier at the grain boundaries thereof.
Still another object of the present invention is to provide a method for fabricating a semiconductor device comprising a semiconductor film having a low barrier at the grain boundaries thereof.
These and other objects of the present invention have been attained by the use of a semiconductor film having a lattice distortion therein and comprising crystal grains having an average diameter of from 30 xc3x85 to 4 xcexcm, preferably from 30 xc3x85 to 400 xc3x85, as viewed from the upper surface of said semiconductor film, and containing an oxygen impurity at a concentration of 7xc3x971019 atoms.cmxe2x88x923 or lower, preferably 1xc3x971019 atoms.cmxe2x88x923 or lower, inside said semiconductor film. Such a semiconductor film include a microcrystalline semiconductor film having a microcrystal structure and a polycrystalline semiconductor film. Microscopically, the crystals (grains) in the semiconductor film are more tightly brought into contact with the neighboring crystals (grains) by imparting lattice distortion to each of the crystals (grains). This results in the disappearance of barriers at the grain boundaries and in an enhanced carrier mobility.
The semiconductor film according to the present invention is different from a polycrystalline film free from lattice distortion, since in the latter film, impurities such as oxygen accumulate at the grain boundaries to develop a barrier which interferes the transfer of carriers. The film according to the present invention has no or negligible amount of barriers therein since the film has lattice distortion therein. Thus, the electron mobility of the film according to the present invention is extremely increased, as compared with that of a conventional semiconductor film, to from 5 to 300 cm2/Vsec.
The concentration of hydrogen in the semiconductor film according to the present invention is preferably 5 atom % or less.
The semiconductor film according to the present invention is fabricated by first depositing an amorphous (or substantially amorphous) semiconductor film on a substrate by sputtering in a 100% hydrogen atmosphere or in an atmosphere containing hydrogen as the major component and an inactive gas, and then crystallizing said amorphous semiconductor film in the temperature range of from 450 to 700xc2x0 C., typically at 600xc2x0 C.
The present inventors have found through an extensive study that, by carrying out the sputtering process in a controlled atmosphere containing 0.01% or less oxygen and having added therein 10% or more (in terms of partial pressure) of hydrogen [having 5N (99.999%) or higher purity], an a-Si film having uniformly incorporated therein hydrogen could be deposited, and that this film was capable of being thermally crystallized by annealing in the temperature range of from 450 to 700xc2x0 C., typically at 600xc2x0 C. The present invention has been accomplished based on those findings.
In accordance with one aspect of the present invention, an amorphous semiconductor film containing oxygen impurity at a concentration of 7xc3x971019 atoms.cmxe2x88x923 or less is deposited by sputtering a semiconductor target containing oxygen impurity at a concentration of 5xc3x971018 atoms.cmxe2x88x923 or less, and the resulting film is then thermally crystallized.
In accordance with another aspect of the present invention, an amorphous semiconductor film is formed by sputtering a semiconductor target containing oxygen impurity at a concentration not higher than 5xc3x971018 atoms.cmxe2x88x923 in a chamber the inner pressure of which is adjusted to a suitable pressure for sputtering, preferably, in the pressure range of from 10xe2x88x922 to 10xe2x88x924 Torr, by introducing a gas consisting of 100% hydrogen or a mixed gas comprising hydrogen and an inactive gas into a chamber having evacuated to a pressure of 1xc3x9710xe2x88x926 Torr or lower, preferably to 1xc3x9710xe2x88x929 Torr or lower, and the resulting film is then thermally crystallized.